Cutting corundum rod



Nov. 8, 1949 M. H. BARNES 2,487,091

CUTTING CORUNDUM ROD Filed July 15, 1944 I U) K i so .1 so :1 m 40 a, I3 2O 0 IO 4o 6O a0 OPTIC ORIENTATION-DEGREES INVENTOR MALCOLM H. BARNESBYM ATTORNEY Patented Nov. 8, 1 949 UNITED STATES PATENT. OFFlE zaa'zmroo'rrmc OOBUNDUM ROD Malcolm 1!. Bones, Kenmore, N. Y., assignor to TheLinde Air Products Company. a corporation of Ohio Application my it,messes: No. 545,011

,15 cam. (01. 125-12) bycuttingandgrindingsoastoimprovethe' yield ofusable products obtained. Primarily, my method is concerned withincreasing the yield of usable products obtained when cutting thinslices from such corundum rods. In addition, my invention pertains tothe marking of unicrystalline corundum rods in a novel manner so that arod can be positioned properly for cutting.

It has been customary to cut jewel bearing blanks from unicrystallinesynthetic corundum boules of large diameter, such as about threefourthsinch, which may be white sapphire, blue sapphire, or ruby, depending onthe presence or absence of coloring matter with the alumina of theboule. Such a boule is first split longitudinally along a central planeeither by nicking the crown or by pinching the loot at the bottom of theboule, and the half-boules are then sliced transversely by a saw to formrelatively large slices which are cut again and ground to producebearing blanks of the desired size. In another procedure, thin rods arefirst cut fromthe halfboules and then sliced transversely. Both of theseprocedures are expensive, laborious, and.

wasteful of the large quantity of raw material which is removed by thesaw in making the many necessary cuts.

Recently, there has been developed a method for growing unicrystallinesynthetic corundum bodies in the form of elongated cylindrical rods ofsmall diameter which can be controlled to approximate the desireddiameters of difierent types of jewel bearing blanks. The rods as grownare clear, homogeneous, somewhat translucent in appearance due to theexistence of minute crystal projections on their surfaces, and are quitestable when their diameters do not exceed three-sixteenths inch. Rods ofgreater diameter are usually unstable and tend to crack and split. Inconventional boules the splitting plane contains the axis of growth sothat rods machined from a halfboule obviously do not have the growthaxis within their boundaries. In rods as grown by the recently developedmethod the growth axis is arranged substantially centrally within therod. When stablesynthetic corundum rods were first made, theirsuitability for jewel bearings was immediately recognized, but upon at-3 lowerthan had been expected. From some rods,

.which were selected at random from a group of apparently identicalrods, excellent yields were obtained, whereas from other rods, very poortempting to fabricate them into jewel bearing 'blanksby slicing themtransversely of their longitudinal axes with a saw, or upon'attemptingto grind them in a centerless grinder, it was found that the averageyield of usable product was much yields were obtained.

After extensive research. into the subject of cutting and grindingsynthetic unicrystalline corundum rods I found, surprisingly, that theyield varied greatly with variations in the optic orientation, and thatthe reason behind the unpredictable results obtained from apparentlyidentical rods was the existence of. different optic orientations in thecrystals. 'Good to excellent yields were obtained upon cutting orgrinding rods optically oriented within ,a certain range, whereas verypoor' results were obtained upon rods oriented outside of this range, asdiscussed in detail hereinafter. This feature of my invention originallywas disclosed and claimed in my application Serial No. 503,240, filedSeptember 21, 1943, of which the present application is a.continuation-in-part.

Further research has shown that good cutting results depend not onlyupon the optic orientation of the rod being cut, but also upon theposition of the optic ax s with respect both to the saw and to thedirection of saw motion.

Among the objects of my invention, therefore, are to provide a novelmethod for improving the yield of usable products obtained bymechanically working unicrystalline corundum rods, as by cutting andgrinding, which involves selecting rods having preferred opticorientations; which involves so arranging such a rod with respect to acutting or grinding tool that the optic axis of the rod assumes-apredetermined preferred position relative to the tool: and whichinvolves so arranging such a rod with respect to a cutting or grindingtool, that the fo ces applied to such rod by the tool are such as in ivea good yield of usable product.

The above and other objects of the invention, and the best manner forattaining them, will become apparent from the following detaileddescription having reference to the annexed drawing. wherein:

Fig. l is an enlarged vertical midsectional view of a unicrystallinecorundum rod illustrating the meaning of optic orientation;

Fig. 2 is a graph showing the relation between the optic orientation ofcorundum rods and the percentage yield of usable jewel bearing blankscut from such rods;

Fig. 3 is an enlarged perspective view of a unicl'ystalline corundum rodmarked in accordance 3 with my invention, and illustrating the meaningof various terms used in describing my method. Fig. 4 is an endelevational view showing a circular saw in position for slicing bearingblanks from a corundum rod by plunge cutting;

Fig. 5 is a diagrammatic view in perspective showing a circular saw inthe process of plunge cutting a slice from a corundum rod, andindicating the character of the forces applied to the rod by the saw;

Fig. 6 is a diagram analyzing the forces exerted by the saw on the rod,and their relation to the optic axis during a cutting operation; and

Fig. 7 is an end view showing diagrammatically a circular saw inposition for cutting a slice from a corundum rod in such a way that therelative translational movement is eifected along a nondiametrical chordof the saw.

Extensive research and experimentation on the mechanical working ofsynthetic unicrystalline corundumrods not exceeding a diameter ofthree-sixteenths inch, as by cutting or grinding them, have shown thatthere exists an important correlation between the crystallographicorientation of such rods and the yield of usable prodexerts the greatestinfluence on the results obtained in cutting or grinding, but theorientation of the a-axes also probably exerts an as yet undeterminedeffect on the results.

As shown in Fig. 1, the optic orientation of a synthetic unicrystallinecorundum rod II is expressed in degrees as the angle 0 between thelongitudinal or growth axis gg of the rod and the optic or c-axis 0-0 ofthe rod, the latter of which lies in the direction in which light can bepassed through the crystal without being doubly refracted. In a singlecrystal of synthetic corundum, which crystallizes in the hexagonalsystem, there is only one such direction, and no light ray travelingthrough the crystal in a direction parallel to 0-0 will be doublyrefracted.

When fabricating synthetic unicrystalline corundum rods, either clear orcolored, by abrading material therefrom as in a sawing or grindingoperation wherein pressure is applied transversely of the longitudinalaxis of the rod, high yields of usable products such as bearing blanksare obtained from rods wherein the optic orientation is between and 80,and preferably between 40 and 80, whereas poor yields are obtainedoutside of these ranges because the rods often crack and split. When theoptic orientation is less than 30, synthetic corundum rods even maysplit spontaneously before any attempt at fabrication has been made but,in any event, splitting or cracking almost invariably occurs when therods are sawed or ground. Rods wherein the orientation is above 80 havemany lateral notches, formed during rod growth, which frequently causelateral cracking during grinding. Moreover notched rods require anexcessive amount of grinding to eliminate the notches.

Since the optic orientation of a synthetic corundum rod changesgradually as the rod increases in length if the initial orientation isless than 90 when grown by the process described in my copendingapplication Serial No. 503,240, it has thus far been impossible toobtain a rod which has absolutely uniform optic orientation throughoutits length unless the initial orientation is 90. As a practical matterthe optic orientation change in a rod of the short 3 feet commonlygrown, and of the preferred optic orientation of'30" to 80, ispractically negligible, so that if the orientation of the por- That isto say, within the broad optic orientation range of 30 to there may be anarrower preferred range within which the corundum rod of modifiedcomposition gives the best results.

The'critical nature of the effect of optic orientation on thefabrication of unicrystalline corundum rods is graphically illustratedin Fig. 2, in which the percentage yield of usable jewel blanks cut oneat a time from synthetic white sapphire rods is plotted against theoptic orientation in degrees. at some orientations were slightly aboveor below the curve of Fig. 2, which represents a fair average of theresults obtained. As shown by the curve, a good yield of usable productwas obtained with synthetic corundum rods optically oriented between 30and 80, and a still better yield was obtained in the preferred range of40 to 80.

The critical optic orientation range for cutting corundum rods holdstrue for all cutting procedures known to me, including those wherein therod is rotated and wherein it is not rotated, as

well as for both single slicing technique and gang sawing techniquewherein many slices are cut simultaneously. Furthermore, this range iscritical for the cutting of circular slices and elliptical slices fromrods.

To recapitulate, one phase of my novel method for increasing the yieldof usable product obtained by mechanically working unicrystallinecorundum rods comprises selecting for fabrication those rods wherein theangle between the c-axis 0-0 and the longitudinal axis g-g is between 30and 80, and preferably between 40 and 80, and mechanically working theselected rods by applying thereto a tool such as a saw or grinder. Sincethe novel method of growing synthetic corundum rods of predeterminedcrystallographic orientation described in my above mentioned copendingapplication makes it possible to.

confine within any desired range the optic orientation of all rodsgrown, it is apparent that by growing rods within the preferred rangesmost of the raw material appears in the final product, and very littleis wasted.

I have also found that the yield of usable prod-- not obtained bycutting slices from a synthetic corundum rod with any suitable type ofcutting device, such as a rotating circular saw, can be greatlyincreased by mounting the rod in such a position with respect to the sawthat, of the total force applied by the saw to the rod, the forcecomponent parallel to the c-axis of the rod is approximately the minimumpossible at any position of the rod with respect to the cutting device(ideally zero). In general, it can be stated that the force componentparallel to the c-axis should not exceed 40% of the total force appliedby the saw summed vectorially. Relative translational movement betweenthe rod and the cutting device toward one another is then eifected in adirection across the rod by moving the cutting delength of 2 or Ofcourse, the actual results of cutting The rod I I and saw I9 of Fig. 4are shown in perspective in Fig. 5 to illustrate the relative magnitudeand direction of the forces applied by the saw to the rod in plungecutting. There are three mutually perpendicular forces acting upon therod as it is being cut, these being a downward force D due to thedownward movement of the saw, a longitudinal force L due to the wobbleof the saw, and a transverse force T due to the frictional drag of thesaw against the rod. Although the saw I9 exerts longitudinal forces intwo opposite directions on the two pieces of the rod I I on oppositesides of the cut, it is necessary only to consider the force toward theend from which the thin slice is being cut, because the force in theopposite direction is exerted on the well supported main body of the rodand is not effective to break or crack the thin slice being removed. Thedownward force D is roughly three times as great as the transverse forceT although the mesh of diamond powder used may affect this ratio. Thelongitudinal force L does not bear a direct relation to D and T, butdepends on the planeness of the saw, the tightness of the bearings, andsimilar factors affecting wobble.

In plunge cutting with gang saws the longitudinal force L is lesssignificant because there are always two saw blades on opposite sides ofeach slice supporting the slice against breakage. This tends to meet theprincipal criterion of this invention bykeeping at a minimum the forceparallel to the c-axls and directed toward the slice being removed.

Referring back to Figs. 3 and 4, the proper positioning of the rod IIwith respect to the saw I9 will be described. Actual experiments onplunge cutting have shown that an excellent yield of usable products inthe form of thin jewel bearing blanks is obtained when the translationalmovement of the saw I9 relative to the rod II is in a direction betweenOand 30 to the normal to the plane I3. It is apparent that thiscondition can also be expressed in these words: that the plane I3 makesan angle 17, measured in a specific direction as indicated in Fig. 3, ofbetween 0 and 30 with the line EW when the latter is normal to thedirection of relative movement.

The results of numerous plunge cutting tests on unicrystalline corundumrods having various orientations O, and positioned at different anglesb, are tabulated below. Yields of circular usable jewel blanks areexpressed as percentages of the total number of slices cut in eachinstance. Approximately the same number of test cuts were made at allthe different combinations of optic orientation 0 and angle ofinclination b.

Results of cutting tests Per Cent Per Cent

Per Cent in yields, it is apparent that both sets of test resultssupport the 60 to range of optic orientation within which the bestresults are obtained.

v In off-center cutting wherein the relative movement between a rod IIand a saw I9 is along a non-diametrical chord 3i of the saw, as shown inFig. 7, the direction of progression 33 of the cut into the rodconstantly changes as the depth of the cut increases and graduallyapproaches the normal 35 to the plane I3. However, the principlesdiscussed previously herein apply, and experience has shown that thebest cutting results can again be obtained by so positioning the rod Iithat the normal to the plane I3 makes an angle a with the constantlychanging direction of progression 33 of the cut, which angle a remainswithin the 0 to 30 range as the cut progresses.

I have described the principles of my invention above as applied tospecific cutting and grinding procedures by way of illustration only. Itis apparent that changes in procedure can be made by those skilled inthe art within the scope of the invention as defined in the appendedclaims.

I claim:

1. A method for increasing the yield of usable product obtained bymechanically working by abrasion unicrystalline synthetic corundum rodsgrown in rod form, which method comprises selecting for fabricationthose rods wherein the angle between the c-axis and the longitudinalaxis is between 30 and 80 degrees, and mechanically working the selectedrods by abrading material therefrom with a tool such as a saw orgrinder.

2. A method for increasing the yield of usable product obtained bymechanically working by abrasion unicrystalline synthetic corundum rodsgrown in rod form, which method comprises selecting for fabricationthose rods wherein the angle between the c-axis and the longitudinalaxis is between 40 and 80 degrees, and mechanically working the selectedrods by 'abrading material therefrom with a tool such as a saw orgrinder.

3. A method for increasing the yield of usable Iv product obtained bymechanically working by abrasion unicrystalline synthetic corundum rodsgrown in rod form and having a diameter not exceeding three-sixteenthsinch, said rods having their growth axes substantially centrallytherein,

said method comprising selecting for fabrication those rods wherein theangle between the c-axis and the longitudinal axis is between 30 and 80degrees, and mechanically working the selected rods by abrading materialtherefrom with a toolv and effecting relative movement between said rodand said saw in a direction across said rod.

5. A method for cutting a slice from a unicrystalline corundum rodcomprising mounting said rod in such a position with respect to a sawthat of the total force applied by the saw, the force component parallelto the c-axis of the rod and directed toward the slice being removeddoes not exceed 40%; and efiecting relative movement between said rodand said saw in a direction across said rod.

6. A method for cutting slices from unicrystalline corundum rodscomprising selecting such a rod having an angle between its c-axis andits longitudinal axis of between 30 and 80; mounting said rod in such aposition with respect to a saw that of the total force applied by thesaw, the force component parallel to such c-axis and directed toward theslice being removed is approximately the minimum possible at anyposition of said rod with respect to said saw; and effecting relativemovement between said rod and said saw in a direction across said rod.

7. A method for increasing the yield of usable product obtained bycutting a unicrystalline corundum rod with a saw, which method compriseseffecting relative translational movement between said saw and said rodwhile holding said rod in such a position that the normal to the planedefined by the c-axis and the longitudinal axis of said rod makes anangle between and 30 with the direction of progression of the cutthrough said rod.

8. A method for increasing the yield of usable product obtained bycutting with a saw a unicrystalline thin synthetic corundum rod grown asa rod having a diameter not exceeding three-sixteenths inch and havingits growth axis substantially centrally therein, which method compriseseflecting relative translational movement between said saw and said rodwhile holding said rod in such a position that the normal to the planedefined by the optic axis and the longitudinal axis or said rod makes anangle between. 0 and 30 with the direction of progression of the cutthrough said rod.

9. A method for increasing the yield of usable product obtained bycutting a unicrystalline corundum rod with a saw, which method comprisesselecting such a rod having an angle between its tion substantiallynormal to the plane defined by the c-axis and the longitudinal axis ofsaid rod.

12. A method for increasing the yield of usable product obtained byplunge cutting a unicrystalline corundum rod with a saw, which methodcomprises selecting such a rod having an angle between its c-axis andits longitudinal axis or between 30 and 80, and efiecting relativetranslational movement between said saw and said rod in a directon suchthat the normal to the plane defined by the c-axis and the longitudinalaxis of said rod makes an angle between 0 and 30 3 with such directionof movement.

c-axis and its longitudinal axis of between 30 and and eflectingrelative translational movement between said saw and said rod whileholdin said rod in such a position that the normal to the plane definedby the c-axis and the longitudinal axis oi said rod makes an anglebetween 0 and 30 with the direction or progression 01' the'cut throughsaid rod.

10. A method for increasing the yield of usable product obtained byplunge cutting a unicrystalline corundum rod with a saw, which methodcomprises eflecting relative translational movement between said saw andsaid rod in a direction between 0 and 30 to the normal to the planedefined by the c-axis and'thc longitudinal axis oi said rod.

' 11. A method for increasing the yield 01' usable product obtained byplunge cutting a unicrystalline corundum rod with a saw, which methodcomprises efi'ecting relative translational movement between said sawand said rod in a direc- 13. A method for increasing the yield of usableproduct obtained by plunge cutting a unicrystalline corundum rod with asaw, which method comprises selecting such a rod having an angle betweenits c-axis and its longitudinal axis of between 30 and 80, and effectingrelative translational movement between said saw and said rod in adirection substantially normal to the plane defined by the c-axis andthe longitudinal axis oi said rod. Y

14. A method for increasing the yield of usable product obtained bycutting with a saw a unicrystalline corundum rod, which method compriseseflecting relative movement between said rod and said saw in a directionalong anon-diametrical chord of said saw while holding said rod in sucha position that the normal to the plane defined by the optic axis andthe longitudinal axis of said rod makes at all times an angle be-. tween0 and 30 with the constantly changing tive translational movementbetween said saw and said rod while holding said rod in such a position,as indicated by the position or said mark, that the normal to the planedefined by said mark and the longitudinal axis of said rod makes anangle between 0 and 30 with the direction oi progression of the cutthrough said rod.

MALCOLM H. BARNES.

I REFERENCES CITED The following references are of file of this patent:

v UNITED STATES PATENTS Number .Name Date 1,766,037 Dawson Tilliev 24,1930- 1326519 Bailey All. v10,1943 2,355,877

record in the I Le Van Aug. 15, 1044

